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Original article Nutrient efficiency and resorption in Quercus pyrenaica oak coppices under different rainfall regimes of the Sierra de Gata mountains (central western Spain) Juan F. Gallardo a Alejandro Martín b Gerardo Moreno a C.S.I.C., Aptdo 257, Salamanca 37071, Spain b Area de Edafología, Facultad of Farmacia, Salamanca 37080, Spain (Received 8 December 1997; accepted 8 January 1999) Abstract - Nutrient uptake, nutrient resorption and nutrient use efficiency (NUE) were estimated in four Quercus pyrenaica oak coppices situated in the Sierra de Gata mountains (province of Salamanca, central-western Spain). The efficiency (NUE) with which a given nutrient is used depends on several factors. In the oak coppices studied, availability of P, Ca and Mg in the soil was one of the factors governing efficiency. On the other hand, there was a certain independence between soil N and K availability and their plant efficiency; in the case of N this occurred possibly because it is a limiting factor. There was a plant nutritional Ca-Mg imbalance due to soil acidity. Leaf absorption and/or leaching at canopy level would also influence the N and K efficiency. The stand with the most dystrophic soil was the least efficient regarding Mg, and the plot with the most eutrophic soil regarding Ca. All the oak coppices had low N efficiency. Bioelement resorption did not affect the NUE decisively but it seemed to be influenced by leaf absorption and leaching occurring at the canopy level. Higher aboveground production suggested that the stands on granite absorbed greater yearly amounts of N, K and P than those on schist. (© Inra/Elsevier, Paris.) nutrient use efficiency / resorption / root uptake / oak coppice / Quercus pyrenaica / biogeochemical cycles Résumé - Efficience et réabsorption d’éléments nutritifs dans quatre taillis à Quercus pyrenaica suivant un transect pluvio- métrique dans la Sierra de Gata (ouest de l’Espagne). L’absorption d’éléments nutritifs, la réabsorption et l’efficience d’utilisation d’éléments nutritifs (NUE) ont été étudiés dans quatre chênaies (Quercus pyrenaica) de la Sierra de Gatu (province de Salamanque, ouest de l’Espagne). L’efficience d’utilisation de bioéléments (NUE) est dépendante de différents facteurs. Dans les chênaies étu- diées la disponibilité édaphique des éléments nutritifs influe sur l’efficience d’utilisation de P, Ca et Mg. Au contraire, il n’y a pas de relation entre l’efficience de N et K, et la disponibilité édaphique de ces éléments, peut être en raison des réserves édaphiques impor- tantes de N total et de l’acidité du sol qui entraîne une insuffisance pour Ca. L’absorption et le lessivage des feuilles des arbres peu- vent aussi influencer l’efficience de N et K. La station avec le sol le plus dystrophe correspond à la chênaie la moins efficiente pour Mg, tandis que la station la moins dystrophe est la chênaie la moins efficiente pour le Ca. En ce qui concerne N, toutes les chênaies ont une efficience très basse. La réabsorption d’éléments biogènes n’affecte pas la NUE des taillis étudiés, parce qu’elle est influen- cée par les processus d’absorption et le lessivage des bioéléments au niveau de la canopée forestière. Les peuplements sur granit absorbent plus d’N, K et P et produisent plus de litière que les peuplements sur schistes. (© Inra/Elsevier, Paris.) efficience d’utilisation des bioéléments / réabsorption / absorption des racines / taillis de chêne / Quercus pyrenaica / cycles de bioéléments * Correspondence and reprints jgallard@gugu.usal.es 1. Introduction Nutrient use efficiency (NUE) has been defined by Ferrés et al. [17] as the biomass production by plants (in terms of fixed C) per unit of nutrient uptake. NUE appears mostly in the literature with reference to infertile habitats, such as marshes [12], peatlands [7], heathlands [2] or semi-deserts [33]. The efficiency of nutrient use by plants to produce biomass may be an important adaptation to infertile habitats [7]; an increase in NUE in a plant species should be a response to the decreasing soil nutrient availability, but this is not found in general [1]. Furthermore, it is not clear whether the greater NUE observed in oligotrophic soils is a charac- teristic of the species inhabiting them or whether it is a phenotypical response of individual specimens to low nutrient availability [4]. In short-lived plants, biomass production per unit of absorbed nutrient is simply the inverse of the concentra- tion of the nutrient in question in the tissues of the plant. However, in long-lived plants some bioelements suffer resorption (i.e. reabsorption by young tissues of nutrients retranslocated from senescent tissues as mature leaves), which allows the plants to use the same unit of absorbed nutrient to produce several vegetative organs [38], increasing the NUE. Resorption is the repeated use of the same nutrient units and could therefore be a good means of estimating the efficiency of nutrient use; nevertheless resorption has not been found for all the bioelements, but is frequent for N and P. Apart from the probable adap- tive value of efficient resorption, important interspecies differences in resorption indices have been observed. Therefore nutrient concentrations only afford a very approximate idea of the efficiency of nutrient use by for- est species. In these cases, it seems more appropriate to estimate efficiency by measuring net primary production (aerial and underground) per unit of nutrient uptake dur- ing the year. Under controlled conditions, such measure- ments are possible; however, they are not very practical under field conditions [4]. As an alternative, Vitousek [38] defined NUE (see equation later) as the total amount of organic matter return (as litterfall and root return) plus that stored per- manently in the plant (in the wood), divided by the amount of nutrients lost (as litterfall, canopy leaching or by root return) plus the nutrients remaining stored owing to the growth of the vegetation (uptake according to Cole and Rapp [ 10]. An easier method of calculating the NUE (specifically for forests) was proposed by Vitousek [38, 39] as the inverse of the concentration of the nutrient (that is, amount of dry matter in litterfall per unit of the nutrient contained in it). Later, Bridgham et al. [7] used the ratio of ’litterfall production/litterfall nutrient’ as an index of nutrient efficiency (NUE; production per unit of resource uptake), distinguishing it from the resource response efficiency (RRE), defined as the production per unit of available resource. In forests an additional problem is the exact measurement of the availability of the resource [24, 25]. Carceller et al. [8] reported that under nutrient stress conditions (either due to soil oligotrophy and/or to low water availability, giving rise to deficiency symptoms) some plants respond with increased efficiency. Nevertheless, parameters of both total and available soil nutrients are sometimes not correlated to plant nutrient uptake (in both fertile and very unfertile soils), probably because many factors affect nutrient efficiency in the field. Vitousek [38] has pointed out that the literature con- tains many references to litterfall and to the amounts of N, P, Ca, Mg and K returned through litterfall, but little information concerning the amount of nutrients stored in wood [14, 31] and even less about root return [8, 30]. Furthermore, Cole and Rapp [10] and Gallardo et al. [20] have shown that N-, P- and Ca-return to the soil is most- ly achieved through litterfall, while K-return is mainly due to canopy leaching; Mg is intermediate between these two possibilities and varies according to the ecosystem studied. Consequently, it is difficult to com- pare the results on NUE from different studies because the data are obtained from different calculations, depend- ing on previous definitions of NUE and the ecosystems. Blair [5] affirmed that the definition of NUE depends on the ecosystem in question (annual, deciduous, evergreen plants, etc.). Aerts [3] stated that efficiency is also related to nutri- ent resorption by plants; reviewing the literature he found that nutrient resorption is close to 50 % for N and P in some tree species. Del Arco et al. [11] reported that N resorption is a key process through which plants reach maximum efficiency in their use of N. Among the factors assumed to exert some effect on the above-mentioned differences in resorption [16] are soil fertility, soil dryness and those affecting leaf demog- raphy (leaf shedding period, time of residence of nutrient in leaves). When requirements are greater than uptake, the plant must meet the rest of its needs for nutrients by retranslocating them from old organs to new ones. Following this line of thought, Carceller et al. [8] calcu- lated bioelement resorption as the difference between the leaf mineral mass at the end of August minus the poten- tial return of nutrients to the soil through the leaf litter [20]. Significant relationships between leaf nutrient con- centration and soil nutrient availability are reported fre- quently, but Aerts [3] did not find any link between leaf nutrient resorption and leaf nutrient concentration, or soil nutrient availability and leaf nutrient resorption. Regarding the effect of soil fertility on NUE, several theories have been advanced; it seems logical that species found on the sites most impoverished in soil P or N would have higher resorption indices because they would be obliged to retain these elements and reuse them as much as possible, thus favouring more efficient inter- nal recycling [34] and affording the plants a certain inde- pendence from the supply coming from the soil. Paradoxically, species living in highly fertile areas may have very high nutritional requirements, leading them to use nutrients more efficiently too [36]. However, in general, the majority of autochthonous European forests are restricted to areas with poor soils. For example, Gallardo et al. [18] have carried out research on deciduous oak (Quercus pyrenaica Willd.) coppices developed on acid soils with low base and available P contents [37]. Other aspects related with the biogeochemical cycles of these forests [23, 26, 27, 37] and their water balance [28, 29] have also been studied. It could thus be of interest to know the NUE and resorption values in four well-studied, oak-forest ecosys- tems of the Sierra de Gata mountains following a rainfall gradient [19] and to see whether it is possible to find dif- ferences between those values in relation to soil charac- teristics, especially soil pH and biochemical properties. The aim of the present work was first to estimate the NUE (according to Vitousek [38]) and resorption of macronutrients on plots of these deciduous oak (Q. pyre- naica) coppices and then to elucidate which factors gov- ern these processes, taking into account the soil avail- ability of each macronutrient. 2. Materials and methods 2.1. Site description and stand characteristics The study area is located in the El Rebollar district (Sierra de Gata mountains, province of Salamanca, west- ern Spain). The co-ordinates of the area are 40° 19’ N and 6° 43’ W. Four experimental plots of Quercus pyrenaica Willd. coppices were selected (table I) with areas ranging from 0.6 to 1 ha. They were named Fuenteguinaldo (FG), Villasrubias (VR), El Payo (EP) and Navasfrías (NF). Stand ages range from 60 to about 80 years (table I). These coppices were thinned for pasturing (cattle). The climate of the area is classified as warm Mediterranean, characterised by wet winters and hot, dry summers [28], with an average rainfall and temperature (table I) of approximately 1 580 L m -2 year -1 and 10.4 °C for NF, and 720 L m -2 year -1 and 12.9 °C for FG. The dominant soils are humic Cambisols developed over schist and greywackes at NF and VR, and over Ca- alkaline granite at EP and FG [26]. The physical, physic- ochemical, and biochemical properties of the four forest soils are shown in table II; soil samples were taken from the selected modal soil profile at each plot [37]. Tree density (table I) ranges between 1 043 trees ha-1 at the VR plot and 406 trees ha-1 at the EP plot [22, 28]. The plot with the lowest tree density (EP) has the highest mean trunk diameter (25 cm), the greatest height (17 m) and biomass (131 Mg ha-1); the lowest values of these parameters correspond to the VR plot (11 cm, 8.5 m and 63.8 Mg ha-1 , respectively). Aboveground production ranged from 4.1 to 2.6 Mg ha-1 year -1 in FG and NF, respectively [20]. Methodological aspects and data of soil analysis, aboveground biomass, litterfall production (from February 1990 to February 1993), foliar analysis, rainfall distribution, throughfall, water concentrations of bioele- ments, canopy N absorption, annual potential return of bioelements (total nutrients returned to the soil through the litterfall, assuming complete mineralization), etc., have been given by Gallego et al. [22, 23], Martin et al. [26], Moreno et al. [28, 29] and Gallardo et al. [19, 20]. Owing to methodological difficulties, no data on root biomass and below ground production of oak coppices have been obtained. Annual nutrient immobilisation in wood has also been estimated [18]. Exchangeable cations were determined following the neutral ammoni- um-acetate method [26]; available Ca and K using 1 N ammonium acetate as extracting solution [37]; and avail- able P using to the Bray-Kurtz [6] procedure. Some of the important soil characteristics of the stands are shown in table II. 2.2. Methods Each plot was divided into three parts, and in each of the three subplots the same experiments were performed. As a result, data refer in general to a mean of three repli- cates. Standard deviations were only calculated where data are directly determined by chemical determinations. 2.2.1. Estimation of tree uptake (TU) An estimation of the annual, soil nutrient uptake by plants was made. The tree nutrient uptake from the soil was calculated according to the following equation (units in kg ha-1 year -1): where TU is tree uptake of the nutrient considered; LF, litterfall; SG, stem growth; and TF, throughfall (nutrients retained in small branches and bark are difficult to deter- mine). 2.2.2. Calculation of efficiency indices Two efficiency indices, involving different factors, were determined. The first was defined by Vitousek [38] as dry matter of litterfall per unit of nutrient content in litterfall; this index is frequently used for N and P (we also use it for K, for comparative purposes) and is shown in table III as NEI (nutrient efficiency index). The second index determined, GEI (general efficiency index), contains all the terms given by Vitousek [38] except the contribution from roots (not determined in this study) and can therefore be defined by the following formula: where LF is litterfall (referred to as kg dry matter ha-1); SG, stem growth (referred to as kg dry matter ha-1); NR, nutrient returned by litterfall (in kg ha-1); NI, nutrient immobilised by stems (in kg ha-1); and TF, throughfall of the nutrient considered (in kg ha-1). The amount of nutrients absorbed by the leaves at the canopy level [29] is subtracted since these nutrients are of external origin and are not absorbed directly by the roots. 2.2.3. Estimation of the resorption index (Re) Taking into account the theoretical considerations expressed above, and in an attempt to overcome the drawbacks involved in the calculation of resorption, the resorption index (Re) was estimated using the following expression (units in kg ha-1): where MM is leaf mineral mass (sum of the masses of the nutrient considered) calculated by harvesting trees of different diameter classes; NR is nutrient return by leaf litter; and CL is nutrient canopy leaching (sensu stricto). In this estimation only the soil losses brought about by root absorption (without considering the increase in root biomass) and the soil gains through leaf litter and throughfall are considered [19]. Thus, the nutrient leach- ing has also been taken into account in this resorption index, as proposed by Ferrés et al. [17]. 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Original article Nutrient efficiency and resorption in Quercus pyrenaica oak coppices under different rainfall regimes of the Sierra de Gata mountains (central western Spain) Juan. of the factors governing efficiency. On the other hand, there was a certain independence between soil N and K availability and their plant efficiency; in the case of. the definition of GEI. In the oak stands studied, the soil nutrient availability governs efficiency in the case of P and Ca, but not in the case of N and K. Concerning